Power saving driving method of mobile telephone

ABSTRACT

In a mobile phone having a liquid crystal display unit, the entire liquid crystal display unit is displayed in a simple display mode at lest in a non-operating standby mode. In the simple display mode, the entire liquid crystal display unit is driven by reducing the number of gradation levels or by decreasing a liquid crystal driving voltage. By using such a controlling method, the power consumption of the liquid crystal display unit can be reduced in the non-operating standby mode. On the other hand, necessary information such as time and the amount of remaining battery is displayed such that the information can be read.

TECHNICAL FIELD

The present invention relates to a power-saving driving method for adisplay device. Specifically, the present invention relates to apower-saving driving method for a display device which can beeffectively applied to a mobile phone including a thin-film transistorliquid crystal display (TFT-LCD).

BACKGROUND ART

In the field of mobile phones, new models of multifunction are beingdeveloped one after another. Multifunction is a requirement of the time.On the other hand, extending a continuous operation time withoutcharging has been a fundamental requirement. In this circumstance,various power-saving measures are proposed, some of them beingconducted.

In a mobile phone, power is consumed not only during a phone call butalso in a standby mode. The power-consuming part of a mobile phoneincludes a control unit (CPU) for controlling the entire mobile phone, aradio communication unit for performing transmission/reception, and adisplay unit.

For example, in a folding mobile phone, in which the display unit cannotbe seen when folded, the display unit is not driven so as to save powerwhen the mobile phone is folded and is in a standby mode. Since thedisplay unit cannot be seen when the mobile phone is folded,power-saving measures in which the display unit is not driven is veryeffective and practical.

However, in an ordinal mobile phone which cannot be folded, time and theamount of remaining battery should be displayed even in a non-operatingstandby mode. Therefore, most of commercially available mobile phonesare configured such that time and the amount of remaining battery aredisplayed.

In response to such requirements, mobile phones including an STN-typeliquid crystal display unit, which have the following function, havebeen proposed. That is, the entire liquid crystal display unit is notdriven and only time and the amount of remaining battery can bepartially displayed in a non-operating standby mode.

On the other hand, the display unit of mobile phones is expected toshift from an STN-type liquid display to a TFT-LCD in accordance withrequirements of colorization, high-resolution, and moving image. Sincepower consumption will increase in accordance with colorization,high-definition, and adoption of a TFT-LCD, requirement for power savingwill further increase. However, the TFT-LCD uses a different drivingtechnique from that for the STN-type liquid crystal display. Thus, theabove-described power-saving measures for the STN-type liquid crystaldisplay unit cannot be applied to the TFT-LCD. Therefore, under presentcircumstances, only a method as disclosed in Japanese Unexamined PatentApplication Publication No. 10-65598, in which the entire display unitis not at all driven, can be applied in order to save power in theTFT-LCD. In this method, however, time and the amount of remainingbattery cannot be displayed in a non-operating standby mode.

Accordingly, the present invention provides a power-saving drivingmethod for a TFT-LCD display device in which required information can bedisplayed while reducing power consumption in a TFT-LCD in anon-operating standby mode in a mobile phone or the like including aTFT-LCD.

DISCLOSURE OF INVENTION

The present invention provides a power-saving driving method for amobile phone including a liquid crystal display unit which is driven ina detailed display mode with all gradation levels. At least in anon-operating standby mode, the liquid crystal display unit is driven ina simple display mode in which the entire liquid crystal display unit isdisplayed with a smaller amount of electrical power as compared with thedetailed display mode.

In the simple display mode, the entire liquid crystal display unit canbe driven by reducing the number of gradation levels or by decreasing aliquid crystal driving voltage.

As described above, required information such as time and the amount ofremaining battery must be displayed even in the non-operating standbymode in a mobile phone. On the other hand, since the amount of therequired information is small, the information can be displayed in asimple mode. Currently, for example, information is displayed with 8gradation levels or more in an operating mode. However, information suchas time and the amount of remaining battery does not have to bedisplayed clearly or in detail with 8 gradation levels or more.

Specifically, information such as time and the amount of remainingbattery can be adequately read even when the entire liquid display unitis displayed with 2 gradation levels. When the display is colorized,8-color display can be realized by using 2 gradation levels for each R,G, and B. On the other hand, when the entire liquid crystal display unitis driven by decreasing the liquid crystal driving voltage and bydecreasing the difference between gradation levels while the number ofgradation levels is remained to be the same, the brightness decreasesand thus the contrast also decreases. However, the information such astime and the amount of remaining battery can be adequately read.

In this way, by reducing the number of gradation levels for liquidcrystal display, the power consumption in an operation amplifier formingthe analog buffer of the liquid crystal display device can be reduced soas to realize power saving. Also, by decreasing the liquid crystaldriving voltage in the liquid crystal display device, electrical chargesof charging and discharging generated every time the gradation levelchanges can be reduced, and thus power saving can be realized.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a power-saving driving method for a mobile phoneaccording to a first embodiment of the present invention;

FIG. 2 illustrates a power-saving driving method for a mobile phoneaccording to a second embodiment of the present invention;

FIG. 3 illustrates a power-saving driving method for a mobile phoneaccording to a third embodiment of the present invention;

FIG. 4 is a block diagram showing an embodiment in which the entireliquid crystal display unit is driven by reducing the number ofgradation levels in a simple display mode;

FIG. 5 is a circuit diagram showing the configuration of a gradationvoltage generating circuit, and a DIA converter and an output circuitcorresponding to one data line output, in a data line drive circuit forperforming 8-gradation-level display by using 3 bits of digital data;

FIG. 6 is a detailed circuit diagram corresponding to one data lineoutput in the output circuit including a polarity signal;

FIG. 7 is a block diagram showing an embodiment in which the entireliquid crystal display unit is driven by decreasing the liquid crystaldriving voltage in the simple display mode;

FIG. 8 is a block diagram of a circuit corresponding to one data lineoutput of the output circuit when a binary drive circuit is also used asa data line precharge circuit;

FIG. 9 is a circuit diagram showing a specific example of the circuitshown in FIG. 8;

FIG. 10 is a circuit diagram showing another specific example of thecircuit shown in FIG. 8;

FIG. 11 is a timing chart illustrating the operation of the circuitshown in FIG. 10 in the detailed display mode;

FIG. 12 is a timing chart illustrating the operation of the circuitshown in FIG. 10 in the simple display mode; and

FIG. 13 illustrates a power-saving driving method for a mobile apparatusaccording to an embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described withreference to the attached drawings.

FIG. 1 illustrates a power-saving driving method for a mobile phoneaccording to a first embodiment of the present invention. When the powerof a mobile phone in an OFF-state is turned ON, the mobile phone iscontrolled by a monitoring unit (control unit) 11 in the mobile phone soas to be switched to a non-operating standby mode. In this state, aliquid crystal display of the mobile phone is driven in a simple displaymode under the control of the monitoring unit (control unit) 11 in themobile phone.

When the user operates a key of the mobile phone, makes a phone call, orperforms communication, the mobile phone is put into an operating mode,a call mode, or a communication mode under the control of the monitoringunit (control unit) 11. When the mobile phone is put into a mode otherthan the non-operating standby mode, the liquid crystal display is putinto a detailed display mode with all gradation levels under the controlof the monitoring unit (control unit) 11. When the key operation, phonecall, or communication ends, a timer provided in the mobile phone isstarted. Then, after a predetermined time has elapsed, the monitoringunit (control unit) 11 controls the mobile phone so that the mobilephone is put into the non-operating standby mode, where the liquidcrystal display of the mobile phone operates in the simple display mode.

When the liquid crystal display of the mobile phone is in the detaileddisplay mode, a TFT-LCD drive circuit drives the liquid crystal with ananalog voltage corresponding to all gradation levels so that the liquidcrystal display shows all gradation levels. Alternatively, the liquidcrystal may be driven with an analog voltage corresponding to a reducednumber of gradation levels by using frame rate control or the like.

On the other hand, when the liquid crystal display of the mobile phoneis in the simple display mode, the TFT-LCD drive circuit drives theliquid crystal with, for example, two power-supply voltages (forexample, 5 V and 0 V) used in a drive stage. Also, when the liquidcrystal display of the mobile phone is in the simple display mode, theTFT-LCD drive circuit drives the liquid crystal with a drivepower-supply voltage which is lower than the drive power-supply voltagein the detailed display mode. For example, if the drive power-supplyvoltage in the detailed display mode is a power-supply voltage of 5 V,the lower drive power-supply voltage is 3 V.

FIG. 2 illustrates a power-saving driving method for a mobile phoneaccording to a second embodiment of the present invention. The secondembodiment is a modification of the first embodiment and has aconfiguration in which the monitoring unit (control unit) 11 in themobile phone can determine whether a text mode in which only characterinformation and icons are displayed or an image mode in which imageinformation and so on is displayed is set, even when the mobile phone isin a mode other than the non-operating standby mode. In thenon-operating standby mode, the liquid crystal display of the mobilephone operates in the simple display mode, as in the first embodiment.

In the second embodiment, the monitoring unit (control unit) 11 controlsthe mobile phone so that the liquid crystal display of the mobile phoneoperates in the simple display mode in the text mode and operates in thedetailed display mode in the image mode, even in a mode other than thenon-operating standby mode.

For example, when only character information is displayed, the characterinformation can be read without difficulty even if the information isdisplayed in the simple display mode, because the contrast of thecharacters and their background is clear. Also, since icons in a menupage are clearly designed and colored with a few colors, the icons canbe recognized without difficulty even in the simple display mode.Herein, text mode refers to a display mode in which a multi-colordisplay using more than hundreds of colors is not always required. Inthe text mode, information on the display can be easily recognized evenin the simple display mode.

The monitoring unit (control unit) 11 determines whether the text modeor the image mode is set relatively easily by using a software tool fordisplaying an image. For example, when a tool for displaying an image isstarted by using a tool selection page in the text mode, the mode isswitched to the image mode where display with all gradation levels canbe performed. When the tool ends, the mode can be easily returned to thetext mode.

FIG. 3 illustrates a power-saving driving method for a mobile phoneaccording to a third embodiment of the present invention. The thirdembodiment is a modification of the first embodiment. Since the basicoperation thereof is exactly the same as that in the first embodiment,the description of the basic operation will be omitted.

In the third embodiment, in a mode other than the non-operating standbymode, the liquid crystal display of the mobile phone is switched to thesimple display mode when it is detected that the display unit of themobile phone cannot be seen by the user, for example, when anear-contact sensor detects that the mobile phone is touching the ear ofthe user during a phone call. Accordingly, more effective power savingcan be achieved.

FIG. 4 illustrates an embodiment in which the entire liquid crystaldisplay is driven by reducing the number of gradation levels in thesimple display mode, and is a block diagram of a data line drive circuitof the liquid crystal display device provided in the mobile phone.

As shown in FIG. 4, the data line drive circuit includes a frame memory18, a data latch 22, a DIA converter 24, a gradation voltage generatingcircuit 26, and an output circuit 28. Referring to FIG. 4, digital datacorresponding to the display is written in the frame memory 18 inaccordance with an address, the digital data corresponding to eachscanning line is sequentially read out from the frame memory 18 so as tobe transmitted to the data latch 22, a gradation voltage correspondingto the digital data is selected in the D/A converter 24, and theselected voltage is amplified in the output circuit 28 so as to beoutput to a data line. As shown in FIG. 4, in the data line drivecircuit including the frame memory 18, the same data as the previousframe can be read from the frame memory 18 when the same display iscontinuously performed. Therefore, it can be suspended to input digitaldata to the data line drive circuit during that time, and thus the powerconsumption required for data transfer can be reduced. A polarity signalis a signal synchronized with AC driving for preventing deterioration ofthe liquid crystal, and is supplied to the gradation voltage generatingcircuit 26 and the output circuit 28. In the gradation voltagegenerating circuit 26, a gradation level is inverted in accordance withthe polarity signal, and thus an analog voltage corresponding to thepolarity can be supplied to the D/A converter 24 for the same data. InFIG. 4, the output circuit 28 has a configuration peculiar to thepresent invention, which is different from the known art.

According to the present invention, the output circuit 28 includes aknown analog buffer 30 and a binary drive circuit 32 for each data lineoutput S1, S2, S3, . . . . Either the analog buffer 30 or the binarydrive circuit 32 is operated in response to a display mode switchingsignal from the monitoring unit (control unit) 11.

In the detailed display mode, the output circuit 28 receives a displaymode switching signal from the monitoring unit (control unit) 11 so thatthe binary drive circuit 32 is switched to a non-operating mode and theanalog buffer 30 is switched to an operating mode. Each analog buffer 30receives a gradation voltage output from the DIA converter 24 so as tooutput an 8-gradation-level drive voltage to the data line outputs S1,S2, S3, . . . .

On the other hand, in the simple display mode, the output circuit 28receives a display mode switching signal from the monitoring unit(control unit) 11 so that the analog buffer 30 is switched to anon-operating mode and the binary drive circuit 32 is switched to anoperating mode. The binary drive circuit 32 receives the mostsignificant bit of a digital signal output from the data latch 22 to theD/A converter 24 so as to output a binary drive voltage to the data lineoutputs S1, S2, S3, . . . .

FIG. 5 is a circuit diagram showing the configuration of the gradationvoltage generating circuit 26, and the D/A converter 24 and the outputcircuit 28 corresponding to one data line output S1, in the data linedrive circuit for performing 8-gradation-level display by using 3 bitsof digital data with their inverted data. The gradation voltagegenerating circuit 26 has 8-level gradation voltages V1 to V8corresponding to 3 bits of digital data, wherein V1>V2> . . . >V8 orV1<V2< . . . <V8 in accordance with the polarity signal. The D/Aconverter 24 is formed by CMOS switches, selects a gradation voltage inaccordance with a 3-bit digital signal output from the data latch 22,and outputs the selected voltage to the analog buffer 30. Further, themost significant bit D0 of the 3-bit digital signal is supplied to aninverter forming the binary drive circuit 32. In each embodiment of thepresent invention, the most significant bit means a bit for selectingeither a high-voltage side or a low-voltage side of all gradationlevels.

The analog buffer 30 normally requires a steady idling current formaintaining its operation. However, the binary drive circuit 32 does notrequire an idling current if the binary drive circuit 32 is formed by aninverter circuit. Therefore, in the simple display mode, by suspendingthe analog buffer 30 and by operating the inverter 32, power consumptionfor a static current of the analog buffer is not required, and the powerconsumption can be reduced accordingly.

The gradation voltage generating circuit 26 is generally constituted byconnecting resistors in series, the number of resisters corresponding tothe number of gradation levels. By applying a current to the resistors,a gradation voltage can be extracted from an intermediate tap. Further,in the simple display mode, by blocking the gradation voltage generatingcircuit 26 (that is, by suspending the current supply), the powerconsumption in the gradation voltage generating circuit 26 can bereduced.

In FIG. 5, the D/A converter 24 is formed by CMOS switches. However, theDIA converter 24 and the gradation voltage generating circuit 26 can bereplaced by a DIA converter and a gradation voltage generating circuitfor generating gradation levels by using capacitive coupling.

FIG. 6 is a detailed circuit diagram corresponding to one data lineoutput in the output circuit 28 including a polarity signal.

A display mode switching signal controls the operation of a binary drivecircuit 32A according to ON/OFF control of switches 1 and 2, blocks theoutput of an analog buffer 30A according to ON/OFF control of a switch3, and also controls the operation of the analog buffer 30A. Amost-significant-bit signal and a polarity signal are input to anexclusive NOR circuit 34, and the output thereof is supplied to theinput of the inverter forming the binary drive circuit 32A. Therefore,the binary drive circuit 32A outputs a power-supply voltage VDD2 or VSS2to a data line in accordance with the most-significant-bit signal andthe polarity signal. On the other hand, the analog buffer 30A amplifiesthe gradation voltage selected in the D/A converter according to digitaldata and the polarity and outputs the voltage to the data line.

Herein, the detailed display mode is specified when a display modeswitching signal is H, and the simple display mode is specified when adisplay mode switching signal is L. In the detailed display mode, thedisplay mode switching signal becomes H, the switch 3 is turned ON sothat the analog buffer 30A operates, and the switches 1 and 2 are turnedOFF so that the binary drive circuit 32A is suspended and the outputthereof becomes a high-impedance state. The analog buffer 30A may referto the polarity signal as required. For example, when the analog buffer30A includes a plurality of analog buffers, these analog buffers areswitched according to the polarity signal so as to be operated.

In the simple display mode, the display mode switching signal becomes L,the switch 3 is turned OFF so that the output of the analog buffer 30Ais blocked, and the analog buffer 30A is suspended so that the outputthereof becomes a high-impedance state. On the other hand, the switches1 and 2 are turned ON so that the binary drive circuit 32A is put intoan operation mode and the binary drive circuit 32A is driven by themost-significant-bit signal and the polarity signal.

As described above, in the detailed display mode, the output circuit 28stops the inverter circuit forming the binary drive circuit 32A andoperates the analog buffer 30A so as to output a gradation voltageaccording to the polarity to a data line. On the other hand, in thesimple display mode, the output and idling current of the analog buffer30A are suspended, the inverter circuit forming the binary drive circuit32A is operated, and the power-supply voltage VDD2 or VSS2 is output tothe data line according to the most-significant-bit signal and thepolarity signal of digital data.

If the analog buffer requires a data line precharge circuit, the binarydrive circuit may also be used as a precharge circuit. In this case, inthe detailed display mode (display mode switching signal=H), the switch3 is turned ON so as to operate the analog buffer. In addition, theswitches 1 and 2 are also turned ON so as to operate the binary drivecircuit during a period for which precharge is required (prechargeperiod). In the simple display mode (display mode switching signal=L),the switch 3 is turned OFF, the analog buffer is suspended, and thebinary drive circuit is operated during one horizontal period, not onlyduring the precharge period. In this description, precharge means that adata line voltage is changed to a predetermined voltage in advance.

FIG. 7 is a block diagram showing an embodiment in which the entireliquid crystal display unit is driven by decreasing the liquid crystaldriving voltage in the simple display mode. Elements which are the sameas those in FIG. 4 are denoted by the same reference numerals and thecorresponding description will be omitted.

As can be seen by comparing FIGS. 4 and 7, the output circuit 28 doesnot include a binary drive circuit in the embodiment shown in FIG. 7.However, two different liquid crystal driving voltages VDD2 and VDD3 arealternatively supplied to the D/A converter 24 and the output circuit 28through a liquid crystal driving voltage selecting switch 36. The liquidcrystal driving voltage selecting switch 36 is controlled by a displaymode switching signal. Herein, VDD2 is the liquid crystal drivingvoltage used in the detailed display mode and VDD3 is the liquid crystaldriving voltage used in the simple display mode, VDD3 being lower thanVDD2. For example, when VDD2 is 5 V, VDD3 is 3 V. Alternatively, a logicpower-supply voltage VDD1 may be used as VDD3.

In the detailed display mode, an H-level display mode switching signalswitches the liquid crystal driving voltage selecting switch 36 to theliquid crystal driving voltage VDD2 so as to supply the liquid crystaldriving voltage VDD2 to the DIA converter 24 and the output circuit 28.In the simple display mode, an L-level display mode switching signalswitches the liquid crystal driving voltage selecting switch 36 to theliquid crystal driving voltage VDD3 so as to supply the liquid crystaldriving voltage VDD3, which is a low voltage, to the D/A converter 24and the output circuit 28. Accordingly, the brightness decreases andthus the contrast also decreases, but the power consumption can bereduced.

Alternatively, the configurations of FIGS. 4 and 7 can be combined sothat the power consumption can be further reduced.

FIG. 8 is a block diagram of a circuit corresponding to one data lineoutput of the output circuit when the above-described binary drivecircuit is also used as a data line precharge circuit. In other words,FIG. 8 shows an example in which a precharge circuit is used as a binarydrive circuit in the simple display mode, in a drive circuit of a liquidcrystal display device configured such that a data line is precharged atthe beginning of one output period, as disclosed in Japanese UnexaminedPatent Application Publication No. 11-119750 and Japanese PatentApplication No. 11-145768 (in a normal display corresponding to thedetailed display mode of the present invention, although thoseinventions do not have distinction between a detailed display mode and asimple display mode).

The output circuit shown in FIG. 8 includes an analog buffer 30B formedby connecting in parallel a buffer 10 having a strong dischargingability and a buffer 20 having a strong charging ability, and aprecharge circuit/binary driver circuit 32B connected to the output ofthe analog buffer. The analog buffer 30B and the prechargecircuit/binary drive circuit 32B are controlled by a display modeswitching signal, a most-significant-bit signal, and a polarity signal.

When a gradation voltage in a high-voltage side is output in thedetailed display mode, where the display mode switching signal is H, aswitch 1 of the precharge circuit/binary drive circuit is turned ON soas to precharge a data line to VDD2 at the beginning of one outputperiod, and the buffer 10 is operated so as to decrease the data-linevoltage to the gradation voltage. At this time, a switch 2 of theprecharge circuit/binary drive circuit is turned OFF and the buffer 20is suspended.

When a gradation voltage in a low-voltage side is output in the detaileddisplay mode, where the display mode switching signal is H, the switch 2of the precharge circuit/binary drive circuit is turned ON so as toprecharge a data line to VSS2 at the beginning of one output period, andthe buffer 20 is operated so as to increase the data-line voltage to thegradation voltage. At this time, the switch 1 of the prechargecircuit/binary drive circuit is turned OFF and the buffer 10 issuspended. The voltage VDD2 is higher than VSS2.

In the simple display mode where the display mode switching signal is L,the buffers 10 and 20 are suspended and only the prechargecircuit/binary drive circuit is operated. The precharge circuit/binarydrive circuit is operated for one output period, not only for aprecharge period.

Herein, as in the example shown in FIG. 6, the polarity signal and themost-significant-bit signal are synthesized into one control signal viathe exclusive NOR circuit so as to control the analog buffer and theprecharge circuit/binary drive circuit. For example, when both of thepolarity signal and the most-significant-bit signal are in L-level or inH-level, the switch 1 and the buffer 10 can be operated, while theswitch 2 and the buffer 20 are suspended. When one of the polaritysignal and the most-significant-bit signal is in L-level and the otheris in H-level, the switch 2 and the buffer 20 can be operated, while theswitch 1 and the buffer 10 are suspended.

FIG. 9 is a circuit diagram showing a specific example of the circuitshown in FIG. 8. In the circuit shown in FIG. 9, each of the buffers 10and 20 of the analog buffer 30B is formed by a known operationalamplifier circuit having a phase-compensating capacitor. In theoperational amplifier of the buffer 10, an output amplifier stageincludes an N-channel transistor 40 and a constant current source 42.The charing function depends on a current controlled by the constantcurrent source 42, but the discharing function is performed by theN-channel transistor 40 so that the operational amplifier can beoperated at high-speed. On the other hand, in the operational amplifierof the buffer 20, an output amplifier stage includes a P-channeltransistor 44 and a constant current source 46. The step-down functiondepends on the current controlled by the constant current source 46, butthe step-up function is performed by the P-channel transistor 44 so thatthe operation amplifier can be operated at high-speed. By combining thebuffers 10 and 20 and the precharge circuit/binary drive circuit, ahigh-speed operation can be performed even when the idling current ineach of the buffers 10 and 20 is suppressed to be low, and an analogbuffer consuming a reduced power can be realized.

The buffers 10 and 20 and the precharge circuit/binary drive circuitshown in FIG. 9 are controlled by the display mode switching signal, themost-significant-bit signal, and the polarity signal, as in the exampleshown in FIG. 8. Switches 48, 49, and 50 for blocking an.≢idling currentare provided in the buffer 10 and switches 52, 53, and 54 are providedin the buffer 20. By controlling the ON/OFF state of the switches, theoperation and non-operation of each buffer are controlled.

FIG. 10 is a circuit diagram showing another specific example of thecircuit shown in FIG. 8. In the circuit shown in FIG. 10, each of thebuffers 10 and 20 of the analog buffer 30B is formed by a drive circuitdisclosed in Japanese Patent Application No. 11-145768. Each of thebuffers 10 and 20 has a configuration using a source follower operationof a transistor. By combining the precharge circuit/binary drive circuit32B, a high-speed operation can be performed even when an idling currentin each of the buffers 10 and 20 is suppressed to be low, and an analogbuffer consuming a reduced power can be realized.

In the buffer 20, a switch 111 is connected between VDD2 and a commongate of NMOS transistors 101 and 102 in order to precharge the commongate of the NMOS transistors 101 and 102, and a switch 112 is connectedbetween an output terminal T2 and VSS2 in order to precharge the outputterminal T2. The drain of the transistor 101 is connected to VDD2through a constant current source 103 and is also connected to its owngate. Also, a switch 121 for blocking the drain-source current of thetransistor 101 is connected between the source of the transistor 101 andan input terminal T1. A constant current source 104 and a switch 122 areconnected in series between the input terminal T1 and VSS2. The sourceof the transistor 102 is connected to the output terminal T2, a switch123 for blocking the drain-source current of the transistor 102 isconnected between VDD2 and the drain of the transistor 102, and aconstant current source 105 and a switch 124 are connected in seriesbetween the output terminal T2 and VSS2. Currents controlled by theconstant current sources 103 and 105 are I11 and I13, respectively.

In the buffer 10, a switch 211 is connected between VSS2 and a commongate of PMOS transistors 201 and 202 in order to precharge the commongate of the PMOS transistors 201 and 202, and a switch 212 is connectedbetween the output terminal T2 and VDD2 in order to precharge the outputterminal T2. The drain of the transistor 201 is connected to VSS2through a constant current source 203 and is also connected to its owngate. Also, a switch 221 for blocking the drain-source current of thetransistor 201 is connected between the source of the transistor 201 andthe input terminal T1. A constant current source 204 and a switch 222are connected in series between the input terminal T1 and VDD2. Thesource of the transistor 202 is connected to the output terminal T2, aswitch 223 for blocking the drain-source current of the transistor 202is connected between VSS2 and the drain of the transistor 202, and aconstant current source 205 and a switch 224 are connected in seriesbetween the output terminal T2 and VDD2. Currents controlled by theconstant current sources 203 and 205 are I21 and I23, respectively.

In the circuit shown in FIG. 10, the operation and non-operation of theswitches 112 and 212 and the buffers 10 and 20 are controlled by themost significant bit of a digital signal and a polarity signal, asdescribed above. In the detailed display mode, when a gradation voltagein a high-voltage side is input as Vin, the switch 112 and all theswitches in the buffer 10 are kept to be OFF during the output period.When a gradation voltage in a low-voltage side is input as Vin, theswitch 212 and all the switches in the buffer 20 are kept to be OFFduring the output period.

FIG. 11 is a timing chart illustrating the operation of the circuitshown in FIG. 10 in the detailed display mode. FIG. 11 shows two outputperiods, that is, an output period (time t0 to t3) in which an arbitrarygradation voltage in a low-voltage side is output and an output period(time t0′ to t3′) in which an arbitrary gradation voltage in ahigh-voltage side is output. The operation will be described withreference to FIG. 11. During time t0 to t3, the switches 111, 112, 121,122, 123, and 124 are controlled as shown in FIG. 11, and the switches211, 212, 221, 222, 223, and 224 are turned OFF.

At time t0, the output voltage Vout is precharged by the voltage VSS2.ON the other hand, a voltage V10 at the common gate of the transistors101 and 102 is precharged by the voltage VDD2. The precharge of thevoltage V10 is completed at time t1, and from time t1, the voltage V10changes to a voltage which is shifted from the input voltage Vin by thegate-source voltage Vgs101 (I11) of the transistor 101. Then, thevoltage is stabilized when V10=Vin+Vgs101 (I11). Herein, Vgs101 (I11)represents the gate-source voltage when the drain current is I11. Theprecharge of the output voltage Vout by the voltage VSS2 which hasstarted at time t0 is completed at time t2. From time t2, the outputvoltage Vout changes to a voltage which is shifted from the voltage V10by the gate-source voltage Vgs102 (I13) of the transistor 102. Then, thevoltage is stabilized when Vout=V10−Vgs102 (I13). Herein, both of Vgs101(I11) and Vgs102 (I13) are positive values. By controlling the currentsI11 and I13 so that these values are equal, the output voltage Voutbecomes equal to the input voltage Vin by the above two expressions.Further, the range of the output voltage is VSS2≦c=Vout≦VDD2−Vgs102(I13).

During time t0′ to t3′, the switches 211, 212, 221, 222, 223, and 224are controlled as shown in FIG. 11, and the switches 111, 112, 121, 122,123, and 124 are turned OFF.

At time t0′, the output voltage Vout is precharged by the voltage VDD2at time t0. ON the other hand, a voltage V20 at the common gate of thetransistors 201 and 202 is precharged by the voltage VSS2. The prechargeof the voltage V20 s completed at time t1′, and from time t1′, thevoltage V20 changes to a voltage which is shifted from the input voltageVin by the gate-source voltage Vgs201 (I21) of the transistor 201. Then,the voltage is stabilized when V20=Vin+Vgs210 (I21), The precharge ofthe output voltage Vout by the voltage VDD2 which has started at timet0′ is completed at time t2′. From time t2′, the output voltage Voutchanges to a voltage which is shifted from the voltage V20 by thegate-source voltage Vgs202 (I23) of the transistor 202. Then, thevoltage is stabilized when Vout=V20−Vgs202 (I23). Herein, both of Vgs201(I21) and Vgs202 (I23) are negative values. By controlling the currentsI21 and I23 so that these values are equal, the output voltage Voutbecomes equal to the input voltage Vin by the above two expressions.Further, the range of the output voltage is VSS2−Vgs202 (I23)≦Vout≦VDD2.

If the gradation voltage in a low-voltage side is lower than{VDD2−Vgs102 (I13)} and the gradation voltage in a high-voltage side ishigher than {VSS2−Vgs202 (I23)}, the range of the output voltage can bethe range of a power-supply voltage.

FIG. 12 is a timing chart illustrating the operation of the circuitshown in FIG. 10 in the simple display mode. In the simple display mode,the all switches in the buffers 10 and 20 are kept to be OFF.

In the output period (time t0 to t3) in which an arbitrary gradationvoltage in a low-voltage side is output, the switch 112 is ON and theswitch 212 is OFF throughout the entire period. In the output period(time t0′ to t3′) in which an arbitrary gradation voltage in ahigh-voltage side is output, the switch 212 is ON and the switch 112 isOFF throughout the entire period. That is, the precharge including theswitches 112 and 212 is used as a binary drive circuit.

Various embodiments of a drive circuit have been described. Theembodiments are premised on the configuration as shown in FIG. 4, thatis, the configuration in which a polarity signal is supplied to thegradation voltage generating circuit 26 and the output circuit 28.However, the drive circuit may be configured such that the polaritysignal is supplied to the D/A converter 24 or the data latch 22 insteadof to the gradation voltage generating circuit 26 and the output circuit28 so that digital data is inverted according to the polarity. When thedrive circuit is configured in that manner, the polarity signal is notnecessarily supplied to the output circuit 28. Those skilled in the artwill understand that the drive circuit shown in FIGS. 5, 6, and 8 to 10can be modified so that the polarity signal is not received.

In the above description, the present invention is applied to mobilephones. However, the present invention can be applied to mobileapparatuses including a TFT-LCD display device other than mobile phones.For example, the present invention can be applied to wrist watchesincluding a TFT-LCD display device.

Also, the switching function for the simple display mode and thedetailed display mode of the present invention can be applied to mobileapparatuses including a TFT-LCD display device which do not have a callfunction. In that case, too, power consumption can be reduced.

FIG. 13 shows a power-saving driving method for a mobile apparatusaccording to an embodiment of the present invention. By turning ON thepower of a mobile apparatus in an OFF state, a menu page is displayed ona display unit of the mobile apparatus under the control of a monitoringunit (control unit) 11. Various software tools can be selected on themenu page, and the menu page is displayed again when the tool is ended.Also, a timer is operated, and the power of the mobile apparatus isautomatically turned OFF under the control of the monitoring unit(control unit) 11 after a predetermined time has elapsed in anon-operating state.

When the power of the mobile apparatus is ON, the monitoring unit(control unit) in the mobile apparatus can determine whether a text modein which only character information and icons are displayed or an imagemode in which image information and so on is displayed is set. Themonitoring unit (control unit) controls the mobile apparatus so that theliquid crystal display of the mobile apparatus operates in the simpledisplay mode in the text mode and operates in the detailed display modein the image mode. The liquid crystal display is displayed with allgradation levels in the detailed display mode and is displayed byreducing the number of gradation levels in the simple display mode. Whenonly character information is displayed or when icons of a menu page aredisplayed, a multi-color display using more than hundreds of colors isnot always required. Thus, display can be performed with reduced powerconsumption by reducing the number of gradation levels.

The monitoring unit (control unit) determines whether the text mode orthe image mode is set relatively easily by using a software tool fordisplaying an image. For example, when a tool for displaying an image isstarted by using a tool selection page in the text mode, the mode isswitched to the image mode where display with all gradation levels canbe performed. When the tool ends, the mode can be easily returned to thetext mode.

Further, in mobile apparatuses having a TFT-LCD display unit, includingmobile phones, the simple display mode and the detailed display mode canbe automatically selected under the control of the monitoring unit(control unit). Alternatively, the user can freely select the simpledisplay mode and the detailed display mode. For example, a mobileapparatus may be first set to the simple display mode and the TFT-LCDdisplay unit may be driven in the simple display mode when it is used.Alternatively, the user may switch the mobile apparatus to the detaileddisplay mode every time the user uses the apparatus or while the useruses the apparatus. The setting and switching may be performed by usingan operation button of the mobile apparatus, or may be performed bysoftware. In either case, those skilled in the art can easily realizethe setting and switching.

The liquid crystal driving method in the simple display mode may be thesame as each of the embodiments shown in FIGS. 4 to 6 and 8 to 12.

INDUSTRIAL APPLICABILITY

As described above, according to the present invention, the entireliquid crystal display unit is displayed in a simple display mode atleast in a non-operating standby mode in a mobile phone having a liquidcrystal display unit. In the simple display mode, the entire liquidcrystal display unit is driven by reducing the number of gradationlevels or by decreasing a liquid crystal driving voltage. By using sucha controlling method, the power consumption of the liquid crystaldisplay unit can be reduced in the non-operating standby mode. On theother hand, necessary information such as time and the amount ofremaining battery is displayed such that the information can be read.

1. A power-saving driving method for a mobile phone including a liquidcrystal display unit which is driven in a detailed display mode with allgradation levels, wherein, at least in a non-operating standby mode, theliquid crystal display unit is driven in a simple display mode in whichthe entire liquid crystal display unit is displayed with a smalleramount of electrical power as compared with the detailed display mode,wherein the entire liquid crystal display unit is driven by reducing thenumber of gradation levels in the simple display mode.
 2. The methodaccording to claim 1, wherein, in a mode other than the non-operatingstandby mode, the entire liquid crystal display unit is driven in thesimple display mode while a specific software tool is not operated.
 3. Apower-saving driving method for a mobile phone comprising a liquidcrystal display unit which is driven in a detailed display mode with allgradation levels, wherein, at least in a non-operating standby mode, theliquid crystal display unit is driven in a simple display mode in whichthe entire liquid crystal display unit is displayed with a smalleramount of electrical power as compared with the detailed display mode,wherein the liquid crystal display unit is driven in the simple displaymode when the mobile phone touches an ear of the user.
 4. The methodaccording to claim 1, wherein the entire liquid crystal display unit isdriven by using a binary drive circuit in the simple display mode. 5.The method according to claim 3, wherein the entire liquid crystaldisplay unit is driven by decreasing a liquid crystal driving voltage inthe simple display mode.
 6. A mobile apparatus including a liquidcrystal display unit which is driven in a detailed display mode with allgradation levels, wherein, in a simple display mode, the entire liquidcrystal display unit is driven with a smaller amount of electrical poweras compared with the detailed display mode, and the liquid crystaldisplay unit is driven in one of the detailed display mode and thesimple display mode, wherein the entire liquid crystal display unit isdriven by reducing the number of gradation levels in the simple displaymode.
 7. The mobile apparatus according to claim 6, wherein the entireliquid crystal display unit is driven in the simple display mode while aspecific software tool is not operated.
 8. The mobile apparatusaccording to claim 6, wherein the entire liquid crystal display unit isdriven by using a binary drive circuit in the simple display mode.
 9. Amobile phone comprising the mobile apparatus according to claim
 6. 10.The mobile phone according to claim 9, wherein the liquid crystaldisplay unit is driven in the simple display mode when the mobile phoneis in a standby mode.
 11. A mobile phone comprising a liquid crystaldisplay unit which is driven in a detailed display mode with allgradation levels, wherein, in a simple display mode, the entire liquidcrystal display unit is driven with a smaller amount of electrical poweras compared with the detailed display mode, and the liquid crystaldisplay unit is driven in one of the detailed display mode and thesimple display mode, wherein the liquid crystal display unit is drivenin the simple display mode during a phone call.